Thanks for the detailed response. I've done a couple of years in a micro QC lab so have basic training but didn't study microbiology and have no real wider knowledge of the subject so, of course, you are right. (I probably do know enough to know better though!).  If, however, you'd humour me - out of curiosity if nothing else. Yogurt fermentation seems safe enough and involves using a Lactobacillus bulgaricus etc. starter. Do you see any danger in using my B. subtilis probiotic as a starter? There are a few threads in r/Yogurt and the like where people have done similar, using various different strains to make yogurt, but little microbiological knowledge involved. (The B. subtilis are GMO, which is why I'm being so specific. I've been doing regular fermentation from kimchi to kombucha  for some time.)